In the straddle calculator you’ll find a required field named “ratio” under Calls and Puts.

Ratio means the “amount of options needed to buy one unit of underlying“. Especially in European Options you may find different ratios (the most common are 10 and 100 though). So if you decide to buy options for 100 shares of a stock XYZ and you know that you need ten options to be able to buy one share, you’ll have to enter 10 in this box.